1. Technical Field
The present invention relates to a projection optical system suitable for being incorporated in a projector which performs enlargement projection of an image of an image display element.
2. Related Art
A refraction optical system configured to include a plurality of lenses as a projection optical system for a projector which can perform projection from a short distance and can obtain a large picture plane by having a wide angle of view substantially equal to a half angle of view of 60 degrees, is proposed (see JP-A-2007-147970). However, in a case where a significantly wide angle of view is obtained by the refraction optical system configured only of the lenses, there are drawbacks in that, particularly, a lens disposed on the enlargement side is likely to be enormously increased in side. In addition, when the refraction optical system performs projection at a wide angle of view, it is considered that a large number of lenses are needed in order to correct chromatic aberration of magnification occurring by a negative meniscus lens which is positioned on the enlargement side and has great power.
As a method to solve the drawbacks of the refraction optical system, a refraction optical system formed of a plurality of lenses and a refraction/reflection complex optical system which uses at least one curved reflective mirror have been proposed (for example, see JP-A-2006-235516 and JP-A-2007-079524). In the refraction/reflection complex optical system, since a reflective mirror is used as a final unit which obtains a wide angle of view, the chromatic aberration of magnification is unlikely to occur, compared to the refraction optical system using only the lenses described above.
However, for example, in JP-A-2006-235516, a significantly wide angle of view is obtained using the refraction optical system and a concave mirror; however, the curved mirror needs to be enormously increased in size and the entire length thereof needs to be enormously increased. In addition, in JP-A-2007-079524, for example, the angle of view is about 60 degrees in the eighth example, a mirror size is decreased by combining a concave mirror and a convex mirror. However, similar to JP-A-2006-235516, the entire length needs to be enormously increased. In addition, the F-number is about 3 and it is dark and an optical system using a transmissive liquid crystal is defective in terms of brightness. Moreover, two mirrors have a aspheric surface, which causes difficulty in terms of achieving accuracy and assembly.
As above, in the refraction/reflection complex optical system, an ultra-wide angle of view is obtained but it is difficult to decrease the entire length, and thereby there are drawbacks in that the mirror is increased in size. For example, the system is not suitable for equipment such as a front projector in which portability is important.
In comparison, a system in which a reflective mirror is used as a front projector has been known (JP-A-2008-250296, JP-A-2012-203139, or the like). For example, in JP-A-2008-250296, one or two aspheric lens is disposed before a aspheric mirror, and thereby a compact configuration is achieved; however, in a system having brightness with the F-number of about 1.7, a range of magnification change is narrowed to about 1.2 times. Conversely, in a system having a range of magnification change of about 2 times, it is dark with the F-number of about 1.85. In addition, in JP-A-2012-203139, a positive lens is disposed closest to the mirror side in the refraction optical system, and thereby it is possible to miniaturize the mirror, which enables the entire optical system to be miniaturized. However, since the system is applied only to the F-number of about 1.8, sufficient brightness is not obtained.
Incidentally, in the related art, a projector for the proximity application is usually used by being fixed during installment such as ceiling installment or wall installment, with respect to a fixed screen. However, recently, there is a high demand that not only the projector is disposed upright and performs projection onto a table surface with a relatively small projection size, but also the projector moves to a relatively large room and can be applied to a large picture plane projection. In a case of the large picture plane projection, in order to obtain sufficient contrast even in a relatively bright place, there is a need to use a bright optical system even to a small extent.